Fair weather cumulus have the appearance of floating cotton and have
a lifetime of 5-40 minutes. Known for their flat bases and distinct
outlines, fair weather cumulus exhibit only slight vertical growth, with
the cloud tops designating the limit of the rising air.
Given suitable conditions, however,
harmless fair weather cumulus can later develop into towering
cumulonimbus clouds
associated with powerful thunderstorms.

Fair weather cumulus are fueled by
buoyant bubbles of air, or
thermals, that rise
upward from the earth's surface. As they rise, the
water vapor within cools
and condenses forming cloud
droplets. Young fair weather cumulus have sharply defined
edges and bases while the edges of older clouds appear more ragged,
an artifact of cloud erosion.
Evaporation along the
cloud edges cools the surrounding air, making it heavier and
producing sinking motion (or subsidence) outside the cloud.